This method will be called by Burp when the user invokes a context menu
anywhere within Burp. The factory can then provide any custom context
menu items that should be displayed in the context menu, based on the
details of the menu invocation.
invocation - An object that implements the
IContextMenuInvocation interface, which the extension can
query to obtain details of the context menu invocation.
A list of custom menu items (which may include sub-menus,
checkbox menu items, etc.) that should be displayed. Extensions may
null from this method, to indicate that no menu items are